A substantial number of metal castings are created by pouring molten metal into a sand or investment casting shell mold. Sand casting is typically performed when the article need not have intricate detail or is later machined and the mold is typically made of sand held together with various binders. In investment casting, the mold is typically made of refractory materials bound together with various binders. Investment casting shell molds are used typically when a precision cast article is desired.
An investment casting shell mold is made by first providing a fugitive pattern of the article to be cast. The pattern is made of a material that will be melted or burn away at a later stage in the process of making an investment casting shell mold. The pattern material is most often times wax, hence the process is often referred to as lost wax casting. This pattern is dipped into a slurry having refractory materials forming a coating thereon.
The fugitive pattern is coated by first dipping with prime coat slurry having a controlled composition and rheology. Typically, the prime coat slurry forming the innermost layer of the mold is composed of relatively fine grained refractory materials so that a less porous surface of the mold contacts the metal. In multi-layered molds, the first or prime coat slurry usually has a higher viscosity than subsequent or backup coat slurries and the refractory materials contained therein typically is of finer particle size so as to produce a smoother cast surface. Backup coats are typically produced using coarser grit sizes, fibers, and lower viscosity slurries. After the application of the prime coat, the dipped pattern then receives a stucco coating of dry refractory materials and is gelled and/or gas dried, preferably air dried in a controlled environment of humidity/temperature. The prime coat slurry and optional stucco typically have refractory materials such as alumina, silica, aluminosilicates, zirconium silicate, and ceramics of a controlled particle size range. The slurries have a binder material which often has colloidal silica. After the coated pattern is dried it is subsequently dipped in the same or different slurry and optionally receives another stucco coating and dried again and/or gelled. The coated shell is repeatedly dipped into a slurry and optionally coated with stucco, gelled, and/or dried after each dip. The succession of slurries and optional stuccos may be the same or different materials and are applied until a desired build up of refractory materials are obtained on the pattern. Each slurry is typically of a carefully controlled composition and rheology. Each stucco typically has coarse refractory materials. Several refractory materials, such as fused silica, fused alumina, tabular alumina and fused or sintered aluminosilicates are some examples of refractory materials used in the stucco. Purified and graded natural sands, for example zirconium silicate and quartz sands are sometimes also used. The desired mold is built-up in this fashion with several slurry and stucco repeat coatings until the desired mold thickness is achieved. The wax pattern is finally removed, usually in a steam autoclave, to leave a mold cavity having a desired shape. The resulting “green” or unfired mold is then fired under a precisely controlled heating cycle to increase its strength and to burn off any residual wax.
Investment casting molds need to be dimensionally stable, inert, and to have good thermal characteristics depending on the type of alloy being cast, the geometry of the cast article, and the nature of the metallurgical structure. Different casting techniques or techniques for the solidification of the cast metal result in different metal grain structures in the resulting casting which affects the strength of the resulting article. Often times a precision cast article having thin metal structures within is desired to have a selected metal grain structure, as in the production of turbo charger rotors and turbine blades. Typically, an equiaxed or a directional grain structure is desired. An equiaxed grain structure is typically accomplished by pouring molten metal into a preheated investment casting mold which is then allowed to cool by conduction of heat from the molten metal through each layer of the investment casting mold where the heat radiates from the exterior surface of the mold. The metal solidifies by nucleation and growth at many sites throughout the casting providing an equiaxed grain structure. Directional solidification is where the article is solidified either in polycrystalline form with a structure made up of directionally aligned columnar crystals or it is solidified in the form of a single crystal.
In the production of a precision cast article having thinner metal structures within the thinner casting sections are prone to premature solidification which causes defects such as undesired grain structure, cold-shuts, misruns, shrinkage, and the formation of voids. Therefore, investment casting molds must exhibit proper thermal characteristics such as heat removal rates and controlled cooling stresses to ensure the metal is cooled in a controlled manner. A temperature which is too low, particularly for castings with thin sections can cause premature chilling of the metal. This will result in a loss of molten metal fluidity and local variations in mold temperature causing variable solidification rates, no-fill defects, and/or undesirable metallurgical structures (i.e. little grain growth and/or fine grain structure) in the finished casting.
In the past, it was also felt that bubbles in the investment casting molds were undesirable and to prevent their formation, anti-foaming agents were widely used in the industry in investment casting slurries.
Typically, molds are pre-heated from 1800° F. up to actual metal casting temperature so that the molten metal can best fill the mold cavity without solidifying prematurely and wrapped in a refractory fiber insulating blanket, such as an asbestos and/or a kaolin fiber blanket, to maintain a correct mold temperature during the cooling process. The wrapping of shell molds with an insulating blanket is fairly labor intensive, and has required the use of hand cut and formed fiber insulation blankets which have to be fitted around a mold, all at significant expense. The procedure has varied results and the success of the procedure depends upon the skill of the operator. Small variations can prove critical to the quality of the finished casting. Furthermore, asbestos is associated with serious health hazards which makes it desirable to eliminate its use. To minimize the problems associated with wrapping investment casting shell molds, other solutions of controlling the cooling of the investment casting molds have been sought. The cooling of investment casting molds has been controlled by incorporating in one or more layers of the mold fugitive particles such as cork, wood, plastic or other destructible material which burns out of the shell in the firing process forming voids therein. Heat destructible grains and fibers have also been included in the stucco for forming voids. Another approach that has been taken is introduction of hollow grains or bubbles such as bubbled grains of alumina into the slurry. Problems associated with these approaches have been found to include minimal or negligible effect in the reduction of heat transfer such as in the use of heat conductive alumina and the expense associated with added materials, process steps or complexity, and associated costs. Therefore, there remains a need for an alternative method for controlling or slowing the cooling of a filled investment casting mold.